The present invention relates to an active matrix liquid crystal display and a method of driving the same, and more particularly to such a display and method suitable for the acquisition of a better quality of image.
FIG. 1 shows an equivalent circuit of for one pixel of a liquid crystal display using insulated gate thin film transistors (hereinafter referred to as TFT's). Now assume that the electric potentials of a gate line 11, a data line 12 and a pixel electrode 8 to which the TFT is connected are V.sub.g, V.sub.d and V.sub.LC, respectively. A layer 15 of liquid crystal sandwiched between the pixel electrode and an opposing substrate having an overall transparent electrode formed on the whole of a surface thereof equivalently forms a capacitance C.sub.LC. The potential V.sub.COM of the opposing substrate is constant. A relation between these potentials or voltages are shown in FIG. 2. For simplification, it is assumed that V.sub.COM =0 (V). When a selection voltage V.sub.S is applied to the gate of the TFT, the TFT is turned on and hence a drain current flows so that charges are injected into the liquid crystal capacitance C.sub.LC and the pixel voltage V.sub.LC shortly becomes equal to data V.sub.D. Thereafter, the gate voltage becomes a non-selection voltage V.sub. NS so that the TFT assumes its OFF state. At this time, the pixel voltage V.sub.LC drops by ##EQU1## due to a capacitive coupling of the liquid crystal capacitance C.sub.LC and a parasitic capacitance C.sub.gs caused from the overlap between the gate and source of the TFT shown in FIG. 1, as disclosed in Japanese patent unexamined publication JP-A-59-119328. Here, V.sub.gh is equal to V.sub.S -V.sub.NS. In the turned-off state of the TFT, the charges stored in the liquid crystal are gradually discharged at a time constant which is determined by the resistance R.sub.LC and capacitance C.sub.LC of the liquid crystal and the off-resistance R.sub.OFF of the TFT. After the selection of the other gate lines or the lapse of one period or cycle, the gate voltage becomes V.sub.S again so that the TFT is turned on. This time, a voltage of -V.sub.D is applied to the data line and the pixel voltage V.sub.LC shortly becomes equal to -V.sub.D. When the gate voltage becomes V.sub.NS, a voltage drop of .DELTA.V as has been described above is produced and thereafter a discharging takes place. In this manner, an alternately inverted voltage is applied to the liquid crystal as shown in FIG. 2. To display an image in a grey scale can be realized by changing V.sub.D between 0 and V.sub.DMAX.
In the driving method of the conventional liquid crystal display as mentioned above, any relation between the gate voltage and the data signal voltage with a view to acquire a better quality of image has not been clarified at all.